Electric conductivity of a semiconductor layer provided between two electrodes, namely, a source electrode and a drain electrode in a field effect transistor is controlled by voltage which is applied to a gate electrode. Basically, a field effect transistor is a representative example of a unipolar element, in which electric charge is transported by either a p-type or an n-type carrier (hole or electron).
These transistors can form various switching elements or amplifying elements depending on the combination; therefore, the transistors are applied in various fields. For example, a switching element of a pixel in an active matrix display is given as an application example thereof.
As a semiconductor material used for such a transistor, an inorganic semiconductor material typified by silicon has been widely used so far. However, high-temperature processing is required in order to deposit an inorganic semiconductor material as a semiconductor layer; therefore, it is difficult to use a plastic substrate or a film as a substrate.
On the other hand, when an organic semiconductor material is used as a semiconductor layer, deposition can be performed at relatively low temperature. Accordingly, it becomes possible to manufacture a transistor over not only a glass substrate but also a substrate with a low heat-resistant property such as plastic, theoretically.
As described above, as an example of a transistor using an organic semiconductor material as a semiconductor layer (hereinafter, referred to as an organic transistor), a transistor using silicon dioxide (SiO2) as a gate insulating layer and pentacene as a semiconductor layer is given (Non Patent Document 1; Y. Y. Lin, D. J. Gundlach, S. F. Nelson, T. N. Jackson, IEEE Electron Device Letters, Vol. 18, 606-608 (1997)). According to this document, electric field effect mobility is 1 cm2/Vs, and transistor performance which is equal to amorphous silicon can be obtained even when an organic semiconductor material is used as a semiconductor layer.
In an organic transistor, carriers are transported between a source electrode/ a drain electrode and a semiconductor layer. When there is a large energy barrier at the interface therebetween, transistor characteristics such as electric field effect mobility deteriorate. In order to solve this problem, it has been proposed to use a lithium fluoride layer for an interface between a source electrode/a drain electrode and a semiconductor layer (Patent Document 1; Japanese Patent Laid-Open No. 2003-298056). However, a lithium fluoride layer can be applied only to an n-channel organic transistor; therefore, an organic semiconductor material is limited to n-type. It has also been proposed to dope a semiconductor layer with a conductivity imparting agent (Patent Document 2; Japanese Patent Laid-Open No. 2004-228371); however, there is a problem that a conductivity imparting agent has low chemical stability. Moreover, adhesion between these electrode materials and organic semiconductor materials is important in order to obtain a transistor having excellent durability.
Also, carrier mobility of an n-type organic transistor is generally said to be smaller compared with carrier mobility of a p-type organic transistor. Note that a carrier of a p-type organic transistor is a hole and a carrier of an n-type organic transistor is an electron. When various switching elements or amplifying element are formed by combining organic transistors having a difference in the carrier mobility, a problem is possibly caused to characteristics thereof.
As described above, a source electrode and a drain electrode that can be used for an organic transistor using various organic semiconductor materials, that are chemically stable, and that have excellent adhesion with an organic semiconductor material are required. It is because an organic transistor having favorable transistor characteristics and excellent durability can be obtained by using such a source electrode and a drain electrode.
In addition, a source electrode and a drain electrode function also as a wiring in an organic transistor in many cases; therefore, high conductivity is required. However, few source electrode and drain electrode with the characteristics as described above and high conductivity have been reported.